I don't know if it's interesting for you, but I use my pocket pc (hp hx4700) for this and I use cheap bluetooth gps receiver connected to ppc. After that it's just a matter of programs running on ppc, and yes voice navigation (or turn-by-turn navigation) works nice. But this is in Europe and I never tried it in USA. To be honest, maps are a bit too expensive to buy them for once a year use
I'm using a PDA + GPS receiver combo in the US. It works great. It costs a little more than a standalone GPS but PDA can be used so many different way and I think that was a good option. Also, you can switch software without costing too much (only the cost of software).